docs(ai-chat): add the 4.5.0-rc.8 changelog entry

[ericallam]

Summary

Adds a 4.5.0-rc.8 entry to the AI chat agents changelog covering the chat.agent changes in that release: a new apiClient option on chat.headStart and chat.createStartSessionAction for pointing chat sessions at a different project or environment, the fix for messaging chat agents deployed to a preview branch, and the fix for Head Start handovers when the agent also defines a prepareMessages hook.

All four changes are from #4018.

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Latest commit: 5321059

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[coderabbitai]

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch docs/ai-chat-rc8-changelog

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